1. Field of the Invention
The present invention refers to processes for obtaining a gallium-containing nitride crystal by an ammonobasic method as well as the gallium-containing nitride crystal itself. Furthermore, an apparatus for conducting the various methods is disclosed.
2. Discussion of the Related Art
Optoelectronic devices based on nitrides are usually manufactured on sapphire or silicon carbide substrates that differ from the deposited nitride layers (so-called heteroepitaxy). In the most often used Metallo-Organic Chemical Vapor Deposition (MOCVD) method, the deposition of GaN is performed from ammonia and organometallic compounds in the gas phase, and the growth rates achieved make it impossible to provide a bulk layer. The application of a buffer layer reduces the dislocation density, but not more than to approx. 108/cm2. Another method has also been proposed for obtaining bulk monocrystalline gallium nitride. This method consists of an epitaxial deposition employing halides in a vapor phase and is called Halide Vapor Phase Epitaxy (HVPE) [“Optical patterning of GaN films” M. K. Kelly, O. Ambacher, Appl. Phys. Lett. 69 (12) (1996) and “Fabrication of thin-film InGaN light-emitting diode membranes” W. S. Wrong, T. Sands, Appl. Phys. Lett. 75 (10) (1999)]. This method allows for the preparation of GaN substrates having a 2-inch diameter.
However, their quality is not sufficient for laser diodes, because the dislocation density continues to be approx. 107 to approx. 109/cm2. Recently, the method of Epitaxial Lateral Overgrowth (ELOG) has been used for reducing the dislocation density. In this method the GaN layer is first grown on a sapphire substrate and then a layer with SiO2 is deposited on it in the form of strips or a lattice. On the thus prepared substrate, in turn, the lateral growth of GaN may be carried out leading to a dislocation density of approx. 107/cm2.
The growth of bulk crystals of gallium nitride and other metals of group XIII (IUPAC, 1989) is extremely difficult. Standard methods of crystallization from melt and sublimation methods are not applicable because of the decomposition of the nitrides into metals and N2. In the High Nitrogen Pressure (HNP) method [“Prospects for high-pressure crystal growth of III–V nitrides” S. Porowski et al., Inst. Phys. Conf. Series, 137, 369 (1998)] this decomposition is inhibited by the use of nitrogen under the high pressure. The growth of crystals is carried out in molten gallium, i.e. in the liquid phase, resulting in the production of GaN platelets about 10 mm in size. Sufficient solubility of nitrogen in gallium requires temperatures of about 1500° C. and nitrogen pressures in the order of 15 kbar.
The use of supercritical ammonia has been proposed to lower the temperature and decrease the pressure during the growth process of nitrides. Peters has described the ammonothermal synthesis of aluminium nitride [J. Cryst. Growth 104, 411–418 (1990)]. R. Dwilinski et al. have shown, in particular, that it is possible to obtain a fine-crystalline gallium nitride by a synthesis from gallium and ammonia, provided that the latter contains alkali metal amides (KNH2 or LiNH2) . The processes were conducted at temperatures of up to 550° C. and under a pressure of 5 kbar, yielding crystals about 5 μm in size [“AMMONO method of BN, AlN, and GaN synthesis and crystal growth”, Proc. EGW-3, Warsaw, Jun. 22–24, 1998, MRS Internet Journal of Nitride Semiconductor Research, http://nsr.mij.mrs.org/3/25]. Another supercritical ammonia method, where a fine-crystalline GaN is used as a feedstock together with a mineralizer consisting of an amide (KNH2) and a halide (KI) also provided for recrystallization of gallium nitride [“Crystal growth of gallium nitride in supercritical ammonia” J. W. Kolis et al., J. Cryst. Growth 222, 431–434 (2001)]. The recrystallization process conducted at 400° C. and 3.4 kbar resulted in GaN crystals about 0.5 mm in size. A similar method has also been described in Mat. Res. Soc. Symp. Proc. Vol. 495, 367–372 (1998) by J. W. Kolis et al. However, using these supercritical ammonia processes, no production of bulk monocrystalline was achieved because no chemical transport processes were observed in the supercritical solution, in particular no growth on seeds was conducted.